The ability to store energy efficiently and cheaply would solve one of renewable energy’s greatest challenges. Many renewable resources, such as wind and solar, cannot provide steady energy output. This represents a challenge to distribution networks, which have been designed to be fed with a steady electricity supply from centralized power plants but which encounter problems when supply fluctuates.
Energy storage would allow dispatchers to “flatten” power peaks and “fill” gaps that occur with use of renewable energy. In reality, this means that electricity is stored when too much of it is produced, and consumed later when not enough power is available.
Today, pumped hydropower is the most widely used energy storage technology, although other technologies also are available, including compressed air storage or electrical batteries. But the storage capacity of existing technologies is limited, and researchers and companies are working to develop alternatives. Some of them are really promising; others deserve at least the label “interesting”.
One of the more promising ideas comes from a German research effort between the Fraunhofer Institute for Wind Energy & Energy System Technology (Fraunhofer IWES) and the Center for Solar Energy and Hydrogen Research Baden-Württemberg (ZSW). Their so-called “Renewable Methane” approach is based on using excess electricity from renewable power plants to produce synthetic methane, which can later be used to reproduce electricity.
In a first step, renewable electricity is used to split water into hydrogen and oxygen using electrolysis. Although the production and use of hydrogen as a possible energy storage source has been long elaborated (and largely abandoned because of high water usage and lack of storage infrastructure), the German researchers go one step further. By using methanization, they combine the hydrogen with carbon dioxide (CO2) so that the end product is synthetic methane. The so-called “renewable methane” can be fed into the natural gas grid where it is stored or used for such purposes as producing electricity in gas-fired power plants, for heating, or as a transport fuel.
No large-scale commercial “renewable methane” plant is in operation yet, but a 25-kilowatt pilot plant was built in 2010 in Stuttgart, Germany, by an Austrian firm. Earlier this month, the same company, together with German car manufacturer Audi, started construction of a larger plant with a capacity of 6,300 kilowatts. It will provide drivers of natural gas vehicles with a daily amount of 138,000 cubic feet (3,900 cubic meters) of renewable methane by 2013.
According to the research team, the CO2 that is used for methanization can be extracted from the atmosphere, derived from cement or chalk plants, or captured at biogas plants. What concerns water usage, the research cooperation underlines that it is quite low: 0.287 pounds (0.13 kilograms) of water per kilowatt hour produced are used – an amount significantly lower than that for coal, natural gas, or nuclear power plants. Moreover, roughly half of the water used for the electrolysis is released in the process of methanization, some of which can be reused.
A clear advantage of the Fraunhofer/ZSW approach is that the renewable methane can be stored in the existing natural gas network, which has a huge storage capacity. Countries with developed natural gas infrastructure such as the United States or European countries would be able to store a great deal of energy. Germany, for example, could store the equivalent of the entire country’s electricity demand for a period of several weeks. Pumped hydro storage, in contrast, could provide electricity only for several hours. Moreover, synthetic production of natural gas allows countries that do not have mountainous- enough regions for large pumped hydro storage to build their own storage capacities domestically.
One major drawback to this approach is the significant energy loss involved. The conversion of electricity into methane occurs with an efficiency of only 60 percent (the pilot project that is currently in operation reaches just 40 percent). If the methane is later used in a natural gas power plant to produce electricity, the effiency falls to 36 percent. Pumped hydro storage, on the other hand, stores energy at an efficiency rate of between 70 to 80 percent.
Fraunhofer IWES and ZSW argue that their approach is the only one that allows reasonable large-scale energy storage, making the losses in efficiency less relevant. But one has to ask if it is smart to let renewable energy plants operate at an efficiency of less than 40 percent. (The process efficiency can be increased to around 50 to 60 percent if the methanization plant combines heat and power.)
While the low efficiency of this approach may counteract its convenience and prove to be its undoing, the development of reliable large-scale energy storage alternatives for widespread deployment of renewable energy is necessary. Thus, as long as no true alternatives for long-term energy storage exist, the approach of Fraunhofer Institut IWES and ZSW should be considered as a potential future energy storage technology by investors, the industry, and policy makers.